I know I've read the explanation before, but memory is a fleeting phenomenon as my years add up. But hopefully I'm wiser with time, and not just a wise -a...!
Here's a thought-out guess:
If the motors are toed out, they'll fight each other when going straight and have different turning radiuses in different directions. Both bad characteristics.
If they're lined up parallel, they have the same turning radius, but the one on the outside has to go further to make the turn and the inner one will want to make the boat go straight as it is on the inside of the curve pushing just as hard as the one on the outside. Thus it's hard to turn.
If they're toed in, the slight toe in neutralizes itself in straight travel, and the outside one on a turn has angular leverage over the inside one to force the boat to turn and track in the desired direction.
The explanation may also lie in whether the motors are counter-rotating and which way they're set up: (Function of prop-walk)
C-W on Port, CC-W on Starboard, (more common)
or
CC-W on Port, C-W- on Starboard (less common, higher stern lift)
Dr. Bob or someone has answered the question before. Maybe their explanation will be easier to follow!
This is a bigger issue on catamarans with their greater lateral separation of motors.
Joe. :teeth :thup